Лако-красочные материалы — производство

The shortcomings of the unfilled acrylic resins as luting agents were emphasized in Section II. B.7, and for similar reasons, these clear acrylics have failed to establish themselves as restorative materials. The composite resins on the other hand, after a lengthy development period have come to be recognized as one of the most useful and versatile classes of dental materials now available to the clinician for both anterior and posterior restorations [26]. Composite resins are essentially ceramic-filled, polymer­izable dimethacrylates, the curing (hardening) of which, as pointed out before, involves three-dimensional cross-linking through free-radical polymerization of the acrylic groups, initiated either chemically (i. e., through peroxide-amine redox initiation) or

photolytically (i. e., through a light-activated process commonly involving a-diketone photooxidants and amine-type photoreductants). Contrasted with the unfilled acrylic, the present-day composite resin systems feature low exotherms and comparatively low polymerization shrinkage (typically, 1.5%), low water absorption and solubility, yet improved thermal properties, esthetics, biocompatibility, and mechanical stiffness.

The dimethacrylate resins constituting the matrix generally contain aromatic ring structures to impart rigidity and high viscosity. The most common representative, bis — GMA, a bisphenol A derivative, was introduced in Section II. B.7. Other partly aromatic and highly viscous, yet less hydrophilic dimethacrylates as currently used matrix components, imparting enhanced dimensional stability, are 2,2-bis(4-methacryloy — loxyphenyl) propane (bis-MA) and 2,2-bis[4-(3-methacryloyloxypropoxy)phenyl]propane (bis-PMA). To optimize clinical manipulation, the matrix contains low-viscosity comonomers, including the previously introduced TEGDMA and a large variety of aliphatic and aromatic urethanedimethacrylates. Although the degree of conversion and cross-linking increases with raised concentrations of the low-viscosity monomers, at the same time it causes increased polymerization shrinkage with obvious detrimental effects on adhesion to the tooth material. Although incremental placement of the com­posite, with intermittent partial curing of the individual layers, is being practiced in an effort to minimize contraction on curing, this technique tends to reduce the ultimate fracture toughness within the interface between the layers of the restorative. A recently described method of compensating for contraction during polymerization utilizes ammonia-treated montmorillonite as a low-percentage additive [27]. More pro­mising pointers toward overcoming the polymerization shrinkage problem are found in the excellent work currently performed, inter alia, in the laboratories of Eick [6,28,29] and of Stansbury and Bailey [30] on cyclic monomers consisting of spiro-orthocarbonates, such as the cis-trans isomers of 2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro- [5.5]undecane or similar structures possessing exocyclic polymerizable double bonds. Monomers of this type undergo polymerization with volume expansion, and the reaction can be photoinitiated, for example, with (4-octyloxyphenyl)phenyliodonium hex — afluoroantimonate. Structural design features have been discussed and methods for volume change measurement presented [31]. The presence of exocyclic double bonds may facilitate polymerization, and methacryloyloxy-substituted spiro-orthocarbonates, which also polymerize with volume expansion, offer the potential for copolymerization reactions with conventional resin systems. Further advancement in this field can be expected, and this should contribute significantly to the retention properties of composite materials.

The discontinuous, reinforcing phase of the composites, which on a mass basis constitutes some 50 to 85% of the total cement, consists of siliceous ceramic filler particles, generally crystalline quartz, barium or strontium aluminoborate silica, alumi­nosilicate glasses, prepolymerized composite material, and specialty biphasic glasses. Depending on filler particle size, one distinguishes the conventional composites, with a filler size of 1 to 50 pm, from an important intermediate class of composites featuring 1- to 5-pm filler size, a third class known as microfilled composites with a mean particle size of 0.04 pm, and finally, the so-called hybrid composites, which for most efficient packing and highest fracture toughness, typically incorporate some 70 to 75% of con­ventional filler and 8 to 10% of submicron-size silica filler. These variations of filler type, size, and concentration play a major part in affecting the physical and performance characteristics and thus the optimal clinical conditions for application of each one of the numerous types of compositions on the market.

The strength, fracture toughness, and general durability of the resin-filler combina­tions in the oral environment are all critically dependent on a strong bond between resin matrix and reinforcement particles. Weak interfacial bonding leads to marginal degrada­tion, penetration of oral fluids, and premature wear under the masticatory forces. Untreated filler materials are anchored to the matrix essentially by the micromechanical mode, as the polymerizing resin locks into the surface voids and crevasses of the filler or penetrates into the pores of especially porous filler materials. Introduction of a chemical adhesion component in the form of coupling agents improves the bonding dramatically. The commonly utilized compounds are methacrylate-terminated alkoxysilanes [e. g., 3-methacryloyloxypropyl(trimethoxy)silane], occasionally in combination with zirconates and other co-coupling agents. The rationale behind this structural choice is the expectation that, upon treatment of the filler materials (glassy fillers requiring preetching) with cou­pling agents of this type, silyl ether bonds are formed with surface hydroxyl groups of the filler, while polymerizable vinyl groups protrude from the surface layer and, on com­pounding with the resin, should be available for copolymerization and cross-linking with the embedding matrix. In practice, however, most of the vinyl groups of the silanized filler surface appear to undergo homopolymerization, and the actual resin bonding involves formation of an interpenetrating, rather than cross-linking, network on the inter­face as the polymerizing matrix resin diffuses into the polymethacrylate surface layer. Irrespective of the actual bonding mechanism operative in the interface, silanizing of filler materials prior to compounding with the matrix is generally the accepted method of efficaciously enhancing resin-filler adhesion. Typical diametral tensile bond strength values reported for a light-cured, zirconate-treated bis-GMA resin composite containing a silanized glass filler are 55 to 56 MPa, as against 32 MPa for a composite containing untreated glass [32].